"It means that more than simple genetic changes are necessary," she continues. "It does not mean that H5N1 cannot develop into a pandemic strain."

Gerberding warns against getting complacent about bird flu.

"We are far from out of the woods on H5N1," she says. "Avian influenzainfluenza virus constantly evolves. We don't know where these changes will take us."

Bird Flu in People

The H5N1 virus currently spreads much more easily among birds than among people.

Since 1997, there have been more than 200 lab-confirmed cases of H5N1 infections in people, according to background information in the new study.

The World Health Organization's web site notes 232 reported cases of bird flu in people worldwide since 2003, including 134 deaths, as of July 26, 2006. None of those cases has occurred in North America or South America.

The vast majority of bird flu cases occurred in people who had direct contact with dead birds, note CDC researcher Taronna Maines, PhD, and colleagues.

"Despite limited instances of probable human-to-human transmission, H5N1 viruses have not yet acquired the ability to transmit efficiently among people," the researchers write.

Genetic Remix

Maines and colleagues studied H5N1 transmission in ferrets, which served as a model for people.

First, they gathered genetic material from a 1997 strain of the H5N1 virus and the H3N2 human fluflu virus.

Next, the scientists added a little of the H5N1 genetic material to the H3N2 virus. Basically, they created a viral remix, taking care that the remixed virus couldn't escape.